This video uses your eye's built-in blind spot to trick your brain into making a man's head vanish. I've seen some pretty awesome optical illusion videos in my time, and I'm calling it now: this is the best one yet.

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We've talked about the blind spot before, but a concept this awesome deserves a second look, particularly when combined with such a mind-bending demonstration of the eye's blind spot in action. Since the video is content to simply show the optical illusion in action without explaining it, let's take a closer look at what's going on here.

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This simplified diagram of the human eye gives a good sense of the problem. The photo-receptor cells, which detects the light that makes our vision possible, are found all along the optic disc of the retina, which is #1 in this diagram. These cells connect to the nerve fibers, which run towards the optic nerve at the back of the eye. The problem is in how these structures of the vertebrate eye - the optic nerve actually passes through part of the retina, creating an area with no photoreceptor cells. Hence the blind spot.

One of the most remarkable things about the blind spot is just how good the brain is at hiding its existence. Our blind spot is absolutely gigantic, covering a portion of our field of vision big enough to hold 17 full moons, and yet nobody in recorded history suspected its existence until the 1660s. The fact that the blind spot only affects part of our peripheral vision is obviously also a big part of why it remained undetected - I'm guessing someone would have noticed if it was right in the middle of our field of vision. The presence of a second eye is also crucial, as the brain can use visual information from one eye to fill in the holes in the others' field of vision.

But when the other eye is covered up, as is the case in the video up top, the brain just makes its best guess based on what surrounds the blind spot. That's why the man's head disappears, while the black bar remains solid - the bar is a simple continuous line that the brain knows how to complete, but it can't infer the existence of a person's head simply based on its body. We might imagine a hypothetical evolutionary path in which the brain adapted to deal with the blind spot by filling in the gaps from our memory, which would mean the head would remain visible. Admittedly, that sounds like a recipe for seeing a lot of things that aren't there, which is probably why vertebrate brains didn't evolve that way.

The blind spot was discovered by a French physicist and priest named Edme Mariotte, who discovered this break in the retina while dissecting an eye in the 1660s. He realized that this gap necessarily meant a substantial part of our field of vision was blocked off, completely unbeknownst to all of humanity. Through a series of experiments that basically involved moving his eye around until a fixed point disappeared - a primitive version of the video up top, essentially, he was able to locate just where the blind spot was in our field of vision.

His discover cut against the grain of the accepted wisdom of the time, which had assumed not entirely unreasonably that what we now know as the blind spot would actually be where vision was strongest, since it was right next to the optic nerve. Mariotte is said to have delighted the French court of Louis XIV by playing simple tricks with the blind spot, making coins vanish and reappear to the delight of nobles. This fascination with the blind spot spread to other courts, and there are stories of Britain's Charles II strolling through the royal gardens and closing an eye to make the heads of courtiers disappear. Considering Charles's father was the only British monarch to have his head cut off, that perhaps says some rather disquieting things about the king's sense of humor.

Weirdly, though this video would fool pretty much human eye and those of any other vertebrate, it wouldn't work on an octopus. That's because the eyes of octopuses and all other cephalopods have a completley different structure, in which the optic nerve passes behind the retina, creating an unbroken field of vision. That means the blind spot is really just yet another way that octopuses can feel superior to us, I'm afraid.